Image sizing directives should be removed from alt text

[mattwigway]

Image sizing directives (e.g. width:100px) are specified within the alt text of the markdown image reference. I need my slides to be accessible to visually impaired users, so I need to specify alternative text for all images. Right now, if I do something like ![Graph showing increased carbon emissions height:600px](image.png) the alt text for the image in the HTML slideshow is "Graph showing increased carbon emissions height:600px." It would be nice to strip out the marpit image size directives from the alt text during rendering.